1. Field of the Invention
This invention relates to an apparatus for controlling an emission power of a laser diode, and more particularly to an emission power controlling apparatus which is adapted to carry out power calibration of an optimum recording power of a laser diode used for recording information onto a recordable optical disk such as a R-CD (Recordable Compact Disk), etc.
2. Description of the Related Art
Aside from a CD (Compact Disk) which is conventionally known, there is a R-CD as an optical disk which is recordable on the user side.
In the case of recording information onto such a R-CD by means of a R-CD player, which is adapted to record signals onto the R-CD and reproduce those signals therefrom, a laser diode for recording is used. Whether the recording quality is good or bad in such a R-CD player, depends upon various factors such as physical characteristic and optical characteristic of the R-CD, a wavelength of a laser diode used, and so on. Thus, it becomes very important to calibrate a recording laser power i.e. an emission power of the laser diode, so that its power value is in correspondence with an optimum value.
A calibration method of determining an optimum recording laser power value is carried out by first recording test signals onto a track exclusively used for calibration provided on the R-CD, with the value of a recording laser power changed stepwise every predetermined time on the basis of an ATIP (Absolute Time In Pregroove) Sync (Synchronizing signal), and thereafter by reading and reproducing the recorded test signals. Then, a d.c. component of the reproduced EFM RF signal is eliminated to obtain a recording laser power value at which the amplitude center of the AC coupling reproduce signal is equal to substantially zero. Discrimination of the recording laser power value is carried out by dividing the detected power range and measuring the recording laser power values ever respective divisional points and thereafter by interpolating the recording laser power values between respective divisional points by a method such as a linear approximation, etc. to make adjustment to improve the accuracy.
However, the optimum recording laser power value obtained by the above-described calibration, varies in dependency upon a laser diode wavelength. Further, this wavelength varies in dependency upon a chip or ambient temperature of the laser diode. On the other hand, the wavelength of the laser diode also changes due to the passage of time (chemical change of coloring matter etc.) of the disk.
Consequently, the optimum recording laser power value varies depending upon the ambient temperature and the aging of the laser diode.
As a result, the optimum recording laser power value will significantly vary in dependency upon the ambient temperature and the wavelength of the laser diode. For this reason, even if the above mentioned calibration is performed, an optimum value deviation may certainly exist.
Further, in the case where the laser diode is continuously operated, there is the possibility that the temperature may rise up in great degrees. Accordingly, a true optimum recording laser power value at the time of calibration and that at the time of an actual operation may differ from each other in great degrees.
As a result, it could not be said that the recording operation is necessarily carried out at an optimum value, which is the first problem of the related art devices.
Meanwhile, every time the recording operation onto a disk is interrupted in such a R-CD player, the optimum recording laser power value is determined before resuming the recording operation. For this reason, if the calibration is carried out every interruption of recording in the R-CD player, a considerable calibration area on the disk is required. For example, there may be prepared, as such a calibration area, an area permitting about as much as 100 times of calibrations, which is the second problem of the related art devices.